The present invention generally relates to methods and apparatus for providing excitation for electric machines and, more particularly, improved rotors for electric machines which may operate as starter-generators in vehicles such as aircraft.
A prior art starter-generator may be constructed as a three stage electric machine. A first stage may comprise a permanent magnet generator (PMG); a second stage may be an exciter generator; and a third stage may be a main generator. When the prior art starter-generator is operated in a generating mode, the PMG may provide excitation for the exciter generator and the exciter generator may then provide excitation for the main generator. When the prior art starter-generator is operated in a starter mode to, for example, start an aircraft main engine, excitation for the main generator may be provided from a ground-based power source or from an on-board auxiliary power unit (APU).
It is a continuing goal of vehicle designers to reduce weight and cost of on-board components. In that regard, there is a need to provide for more efficient provision of excitation for a starter-generator. There is a need to reduce requirements for external excitation power during engine starting. There is also a need to eliminate a requirement to provide a PMG to provide initial excitation during power generation.